1. Field of the Invention
The present invention relates to a process for preparing uni-tape prepreg. In particular it relates to a process for preparing uni-tape polymeric prepreg from powder coated filamentary towpregs.
2. Description of the Related Art
The thorough impregnation of carbon fibers with high temperature polyimides is a vital step in producing quality prepreg for the preparation of advanced composites. The quality of a composite part is directly related to the quality of the material precursor from which it was fabricated. A finished composite part that is void free and exhibits good mechanical properties is practically impossible to manufacture if the starting materials for its fabrication (i.e. prepregs) are of poor quality. Thus, the preparation of quality prepregs that meet the required specifications and have uniform fiber/polymer distributions is an essential part of the composite fabrication process.
One of the major problems associated with prepregging is achieving a balance between the structure-property relationships and processing characteristics. In fabricating composite parts, a prepreg's tack and drape characteristics are often considered.
Several processes have been developed in order to make good quality prepreg from high temperature polymers which may have differing tack and drape properties. One approach is to prepare polymer prepregs from monomeric reactants (PMR). The disadvantage to this approach is that it requires the use of monomeric diamines, many of which are toxic.
Solution prepregging is another method which is used for producing prepreg. In this method, the viscosity of the polymer is lowered by adding solvents. It is important that the polymer selected for this process has good solubility characteristics. The amount of tack and drape exhibited by prepregs prepared by this process is a function of the residual solvent remaining in the polymeric matrix and can be controlled depending on the drying capabilities of the process. Thus, residual solvents need to be removed during consolidation which is cause for numerous problems such as toxicity and void formation in the fabrication of large composite parts.
In yet another process, collimated fiber bundles are brought into contact with films of thermosetting monomers that have very low viscosities when heated. The impregnation of the fibers by these hot-melt monomers is achieved easily and good wet-out of the fibers by the resins is made. The low viscosity needed to impregnate fibers with thermoplastic polymers of high molecular weight can be achieved by a number of methods. As described in U.S. Pat. No. 4,541,884 to Cogswell et al., plasticizers may be used. In their process, filaments are drawn through a melt comprised of a polymer and a plasticizer. This process has its disadvantages whereby the plasticizer must be thermally stable at least up to the temperature of the melt and have volatility characteristics such that the plasticizer can be volatilized from the composition below the decomposition temperature of the composition. In addition, the plasticizer must have a sufficiently low volatility at the temperature of the melt to plasticize the polymer in the melt and to give a melt of reduced viscosity compared with the melt viscosity of the polymer alone. Thus, in order to assure optimum property retention, the plasticizer must be removed from the polymer prior to prepreg consolidation into a structural part.
O'Connor (U.S. Pat. No. 4,680,224) describes a pultrusion process for preparing thermoplastic prepregs. In this process, a polymer slurry comes in contact with fiber strands which are subsequently dried in an oven causing the polymer to adhere to the fibers. The fibers are then passed through a tapered die to attain a well consolidated thermoplastic prepreg. In order to aid in processing, wetting agents and anti-corrosion compounds are utilized which remain in the final product.
Attempts have been made to prepare prepregs which have no volatiles remaining inside the matrix resin. These prepregs exhibit no tack and little drape character. In order to process these materials, they must be heated above their glass transition temperatures during processing to provide the desirable tack and drape. One such method involves the passage of grounded reinforcing fibers over an electrostatically charged, fluidized bed of powdered thermoplastic resin. The powder is attracted to the fiber and the resin is flowed into place as the material passes through an inline oven. Muzzy et al. (U.S. Pat. No. 5,198,281) discloses a similar method for the production of a non-woven towpreg fabric. In their process, reinforcing filaments are spread, coated with the matrix forming material to cause interfacial adhesion, and the towpreg plies formed by heating the matrix forming material until the matrix forming material liquefies and coats the reinforcing filaments. The towpreg plies are then cooled in a manner so that the towpregs remain substantially unconsolidated until the matrix forming material solidifes. A plurality of the towpreg plies are overlapped and bonded to together to form multidimensional fabrics, preforms and composite articles. Baucom et al. (U.S. Pat. No. 5,057,338) describes a process for the uniform application of polymer powder particles to a filamentary material in a continuous manner to form a uniform composite prepreg material. A tow of the filamentary material is fed under carefully controlled tension into a spreading unit where it is spread pneumatically into an even band. The spread filamentary tow is then coated with polymer particles from a fluidized bed, after which the coated filamentary tow is fused before take-up on a package for subsequent utilization.
An object of the present invention is to provide a dry powder process for preparing uni-tape prepreg which does not require the use of additives to lower the polymer viscosity.
Another object of the invention is to provide a process for preparing uni-tape prepreg whereby the starting material is a plurality of polymer powder coated filamentary towpregs.
Another object of the invention is to provide a process for preparing uni-tape prepreg wherein a sandwich is prepared from the polymer powder coated filamentary towpregs and a material.
Another object of the invention is to provide a process for preparing uni-tape prepreg wherein pressure is repeatedly applied perpendicularly to the sandwich with a longitudinal oscillating action which allows the filaments to move apart and the polymer to wet them.